This invention relates to a method of removing cured silicone polymer deposits from the surface of electronic components to provide product rework, recovery, and defect repair in microelectronics fabrication. The invention is particularly concerned with a novel and highly efficient method of removing cured Sylgard(trademark) (Trademark of Dow Corning Corp.) and related elastomeric silicone adhesives from the surface of ceramics, metals, cured epoxy resins, and polyimides for reclamation and reuse of the recovered semiconductor assembly parts.
The present invention describes a new method of removing cured elastomeric silicone adhesive, particularly, Sylgard and related silicone polymers which are commonly used in electronic module assembly. Silicone polymers are widely used in microelectronics fabrication processes as sealants and adhesives. For example, one major application in non-hermetic ceramic module assembly includes Sylgard seal band attachment of a protective metal cap onto a ceramic chip carrier to provide protection of the semiconductor device against mechanical damage, moisture ingress, and environmental corrosion.
Other applications of the silicone polymers include: device encapsulation, top seal between the silicon device chip and the substrate to provide an xcex1-particle barrier, passivation coatings on printed circuit boards, coatings on various metallic, plastic, and thermoplastic components to provide protection against mechanical and environmental damage, and use of conductive silicones to attach a heat spreader or a heat sink to the backside of a flip chip for heat dissipation. In addition, thermally and electrically conductive adhesives based on a silicone matrix in conjunction with various types of fillers such as silica, quartz, alumina, aluminum nitride, and metals such as Cu, Ag, Au, silver plated Al, Inxe2x80x94Sn on Cu or Ni, and carbon black find applications as adhesives for direct attachment of heat sinks or heat slugs to device chips for heat dissipation and also as die bond adhesive in wire bonded packages. Commonly used heat slug materials include Alxe2x80x94SiC, anodized Al, SiC, metal matrix composite, Cu and Mo.
Microelectronics fabrication processes often require disassembly of assembled components. Typical reasons include carrying out diagnostic tests, to replace or repair the semiconductor device, or to recover electrically good substrates from test vehicles or early user hardware used to assess product performance and reliability prior to actual product release. Removal processes for various assembly materials must be selective for a particular material set and cause no detriment to the substrate integrity and electrical performance. It is also required that the removal method be environmentally and chemically suitable for use in a manufacturing environment.
Sylgard formulation is a primer-less organosiloxane based two component system comprising a vinyl-functionalized (CH2xe2x95x90CHxe2x80x94) siloxane, typically vinyl-terminated-poly(dimethylsiloxane) as part A, and dihydro-dimethyl siloxane as part B, along with a curing catalyst and inorganic fillers such as silica and quartz. The adhesive composition is prepared by mixing the two components in a specified ratio and the mixture is de-aireated to remove any trapped air bubbles prior to dispensing on the components bonding sites.
The adhesive is applied onto the surfaces to be bonded and the component parts are aligned and assembled followed by curing up to 170xc2x0 C. to 175xc2x0 C. for 45 to 60 minutes or by stepwise cure up to 150xc2x0 C. involving: (a) ramp from 25xc2x0 C. to about 70xc2x0 C. at 2 to 3xc2x0 C./minute, hold for about 90 minutes, (b) ramp up to 150xc2x0 C. at 2 to 3xc2x0 C./minute and hold at 150xc2x0 C. for about 30 minutes.
Equation (I) is an illustration of the Sylgard chemistry in terms of the reactive components and the curing reactions involved. The crosslinking reactions between the precursors are heat-accelerated resulting in a cured hydrophobic polymer of flexible/elastomeric matrix having special stress absorbing properties. These characteristic features of elastomeric silicones are particularly useful for providing protection from moisture ingress and maintenance of adhesive joints between diverse materials having different thermal coefficients of expansion (TCE) under high stress conditions during thermal cycling and other reliability stress test exposures. 
Thermally and electrically conductive silicones are obtained by incorporating conductive fillers such as alumina, silica, aluminum nitride, and metal powders or carbon black for electrical conductivity when necessary. Typically, the adhesive formulation comprises Al2O3 and SiO2/quartz filled polydimethyl siloxane/dimethyl vinyl terminated glycidoxypropyl trimethoxy silane and dimethyl methyl hydrogen siloxane components and a curing catalyst. Electrically conductive alkyl silicone such as methyl silicone and fluorosilicone resins for bonding chips to lead frames may contain metal powder or metal coated inorganic or organic polymer particles.
A major problem in the use of crosslinked elastomeric silicone adhesives such as Sylgard in electronic assembly products has been the difficulty in removing the cured polymer material and obtaining residue-free surfaces for module assembly rework, repair of defective components, and for reuse or recycling of assembly parts. Among the various known methods for removing cured silicones, mechanical scraping of the bulk of the coating followed by media blast and water rinse using pressurized spray, is labor intensive and has additional problems of surface damage and incomplete removal which invariably requires an additional cleaning operation with organic solvents which again does not result in a silicone-free surface. Yet another problem is that it is limited to removal of flat or planar surface residues only.
Another method is the chemical-mechanical removal which is based on the use of a strongly alkaline solution comprising NaOH, KOH, or tetramethylammonium hydroxide (TMAH) in lower boiling alcohols such as methanol, isopropanol, or mixture thereof, which causes a base induced chemical degradation of a xe2x80x94Sixe2x80x94Oxe2x80x94Sixe2x80x94 chain resulting in removal/dissolution of silicone residue from surfaces. Use of a low boiling solvent with strong alkali has chemical safety and flammability issues. There are also concerns about the compatibility of component materials with such high pH aqueous or alcoholic alkaline environments. An alternate method employs organic solvents without any reactive reagent, typically, toluene, dichloromethane, or dimethylformamide, to cause swelling of the silicone polymer which can then be removed by peeling or other mechanical means. This method is not considered practical because of incomplete removal which also requires the manual operation of peeling off the swelled polymer and because the required solvents are unacceptable for industrial applications due to strict regulations on the use of such solvents arising from associated environmental and health issues.
There are a number of solutions proposed by others for stripping cured elastomeric silicone adhesives from various surfaces. Minetti et al. U.S. Pat. No. 3,969,813, the disclosure of which is incorporated by reference herein, describes a high pressure water jet technique to remove room temperature vulcanization (RTV) silicone encapsulant under the chip to lift off the chip by mechanical impact of a directed high pressure H2O jet stream at 12,000 to 20,000 psi pressure. This method, however, leaves silicone residue which is removed by subsequent solvent-based cleaning with isopropanol (IPA).
Corby, U.S. Pat. No. 3,673,099, the disclosure of which is incorporated by reference herein, describes a method for stripping cured silicones and vinyl polymers as polyvinyl cinnamates from substrates using an organic or inorganic base in N-methylpyrrolidone (NMP) with or without another solvent. Specific stripping compositions claimed to be effective for removing methyl-phenyl polysiloxane resins comprise guanidine carbonate or quaternary ammonium hydroxide in NMP and ethylene glycol monomethyl ether.
Miller et al. U.S. Pat. No. 3,947,952, the disclosure of which is incorporated by reference herein, describes a method of encapsulating beam lead semiconductor devices by a multi-step process including a step involving selective removal of an unmasked portion of a silicone resin through a resist mask. The disclosed stripping compositions are comprised of a tetramethyl ammonium hydroxide (TMAH) in 1:2 volume ratio of NMP and isopropanol (IPA) for removing exposed silicone resin after which the resist mask is removed exposing the remaining silicone resin film protection over active areas of the device.
Heiss et al. U.S. Pat. No. 4,089,704, the disclosure of which is incorporated by reference herein, describes a method for removing silicone rubber encapsulating material from microelectronic circuits using methanolic tetramethyl ammonium hydroxide (TMAH) in ethanol or isopropanol (IPA). Specific siloxane polymers are those with methoxy end groups which undergo curing reactions in the presence of moisture in air.
Rubinsztajn et al. U.S. Pat. No. 5,747,624, the disclosure of which is incorporated by reference herein, describes a process for removing silicone coatings and sealants in electrical devices using an M-rich silicone and a catalyst for degradation of polymerized silicones thereby enabling the removal of silicone conformal coatings from surfaces.
Cured organic silicone coatings are also known to be removed by spray solutions containing a 1:1 ratio of methylene chloride and Freon with less than 10% of ethanol addition. Various solvent-based stripping compositions used in the references cited above are not practical for use in manufacturing environment because of the following problems:
(a) Strongly alkaline solutions based on the use of very low boiling solvents such as methanol, ethanol, and isopropanol have the problem of flammability and chemical safety issues for use in manufacturing applications in addition to concerns for electronic component compatibility with high pH solution treatment. Also, these alcohols are classified as Volatile Organic Compounds (VOCs) which are subject to VOC regulations requiring strict control of air emissions by installing special control devices.
(b) Use of ethylene glycol ether solvents such as ethylene glycol monomethyl or ethylene glycol diethyl ether has become highly restricted in industrial processes due to associated human toxicity. This category of solvents are on the toxic release inventory (TRI) list which are subject to strict environmental regulations for hazardous air pollutants (HAPs).
(c) Chlorinated solvents such as methylene chloride are classified as HAPs and thus are under strict environmental regulations which has restricted their use in production processes in recent years. The fluorochlorocarbons or Freons are among the Ozone Depleting Solvents (ODS) which have been banned and their use has been phased-out.
Considering the problems in the prior art cited above on the stripping methods for cured elastomeric silicone adhesives from various surfaces of semiconductor device and packaging substrates, a need exists for an improved method of silicone polymer removal for rework/repair processes in microelectronics fabrication that does not have the problem of assembly components compatibility, environmental hazard, toxicity and flammability issues associated with the methods described in the prior art.
In view of the drawbacks in the silicone residue removal methods of the prior art, a need exists for an improved method in terms of providing more efficient and complete removal, preferably without requiring manual scrubbing or wiping, and which is based on neutral or mildly alkaline solution chemistry such that it is compatible with the various metals including solder alloys, polymers and inorganic materials used in the fabrication of electronic components.
It is therefore an object of the present invention to provide an improved method of removing crosslinked silicone polymers from electronic components for rework, to repair defects, and for reclamation or recovery of usable parts of the assembly products.
Another object of the present invention is to provide a method of removing silicone polymer residue for reclamation of expensive test vehicles which are currently discarded for lack of a suitable silicone residue removal process thereby adding to the overall cost of the product and increasing the waste volume and disposal cost.
A further object of the present invention is to provide an improved method for removing cured Sylgard deposits from various surfaces of electronic modules which is based on non-alkaline or mildly alkaline solution chemistry and which has no environmental and health hazard concerns, and no chemical safety or flammability issues for use in manufacturing operations.
Yet another object of the present invention is to provide an efficient method of removing Sylgard residue and related silicone polymer residues from electronic components which is compatible with a variety of material surfaces including Cu, Cr, Pb/Sn, lead-free solders, polyimide passivation coatings, cured epoxies, ceramic chip carriers and silicon device chips.
These and other purposes of the present invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawings.
The purpose and advantages of the present invention have been achieved by providing a method for removing silicone polymer deposits from electronic assembly component surfaces, interfaces, and under the chip regions of solder joined device to substrate pads, as for example, in the case of flip-chip bonding.
The method comprises the steps of:
(a) providing a first cleaning solution for silicone polymer removal which comprises a quaternaryammonium fluoride (QAF) compound dissolved in a first essentially water insoluble non-hydroxylic aprotic solvent;
(b) submerging the electronic components carrying silicone polymer residue/deposits in the first cleaning solution heated at 40 to 90xc2x0 C., preferably 45 to 60xc2x0 C. and allowing the components to be subjected to the cleaning action by the solution with stirring or agitation for a first predetermined period of time between about 10 to about 90 minutes, depending on the extent of polymer residue and the component surface topography;
(c) removing the assembly components from the first cleaning solution;
(d) transporting and submerging the components in the first solvent rinse bath which comprises a hydrophobic non-hydroxylic solvent, preferably the same solvent as used for the first cleaning solution, and subjecting the components to the solvent rinse, for example, immersion rinse at room temperature to 70xc2x0 C. with agitation, for a second predetermined period of time between about 5 to about 15 minutes, to replace the cleaning solution on the component surface with the solvent;
(e) removing the components from the first solvent rinse bath;
(f) transporting and submersing the components to the second solvent rinse bath which comprises a hydrophilic essentially water soluble solvent, and subjecting the components or parts to the second solvent rinse at room temperature to about 60xc2x0 C. with agitation such as stirring or immersion spray for about 5 to 10 minutes;
(g) removing the components from the second solvent rinse bath;
(h) transporting the components to an aqueous rinse bath and applying a water rinse, preferably deionized water rinse, for example, spray or immersion spray rinse, at room temperature to about 50xc2x0 C. for 2 to 10 minutes;
(i) subjecting the components to an optional step of briefly rinsing with IPA (isopropanol) to replace water on the component surface with IPA to accelerate drying;
(j) drying the components by blowing dry N2 or air on the surfaces and then heating the assembly components at about 90xc2x0 C. to about 120xc2x0 C. for 30 minutes to about one hour, preferably under vacuum to remove adsorbed moisture from the components.
In an alternative solvent rinse process, the assembly components or parts after the first solvent rinse in non-hydroxylic aprotic solvent such as propylene glycol methyl ether acetate (PMA), are transported to a second solvent bath also containing a hydrophobic non-hydroxylic solvent, preferably the same solvent as used for the first cleaning solution and the first rinse solvent such as PMA, and subjecting the parts to the second solvent rinse similar to the first solvent rinse. After the second solvent rinse, the assembly components are transported to a bath containing IPA where the parts are subjected to a spray rinse or immersion rinse with IPA to replace the PMA solvent with IPA, and then dried by blowing dry N2 or air on the surfaces followed by heating the component parts at about 90xc2x0 C. to about 120xc2x0 C. for 30 minutes to one hour, preferably under vacuum.
The first essentially water insoluble non-hydroxylic aprotic solvent candidates of the first cleaning solution are in the category of propylene glycol alkyl ether alkoate selected from the group consisting of propylene glycol methyl ether acetate (PMA), propylene glycol methyl ether propionate (Methotate), di(propylene glycol) methyl ether acetate, ethoxy ethyl propionate (EEP), di(propylene glycol) dimethyl ether (DMM), and other related hydrophobic non-hydroxylated solvents.
The quaternary ammonium fluoride (QAF) compound in the first cleaning solution for silicone polymer removal is represented by a tetraalkylammonium fluoride compound based on the formula R1R2R3R4N+Fxe2x88x92, where R1, R2, R3, R4 are the same or different and are selected from the group consisting of an organic radical CnH2n+1 where n=1-8, where the preferred candidates include tetrabutylammonium fluoride (TBAF), tetramethylammonium fluoride (TMAF), tetraethylammonium fluoride (TEAF), or tetra-n-octylammonium fluoride (TOAF).
The quaternary ammonium fluoride (QAF) used in the first cleaning solution can be in the form of a hydrate represented by R1R2R3R4N+Fxe2x88x92xH2O, where x=3-5, or it can be as an anhydrous solution in tetrahydrofuran (THF), where R1, R2, R3, and R4 are the same as represented by R4N+Fxe2x88x92, where R is a methyl, ethyl, n-propyl, isopropyl, n-butyl, or n-octyl, and combination thereof, or these are different and are selected from the group consisting of an organic radical CnH2n+1 where n=1-8. R is a methyl, ethyl, n-propyl, isopropyl, n-butyl, or n-octyl group.
The preferred quaternary ammonium fluoride (QAF) compound in the first cleaning solution is tetrabutylammonium fluoride (TBAF) which is present at a concentration of about 0.2 to 5 weight %, preferably 0.5 to 1% based on the formula (C4H9)4N+Fxe2x88x92, or 0.6 to 1.5% (weight %) as the trihydrate (TBAF.3H2O) in hydrophobic aprotic solvent, preferably propylene glycol methyl ether acetate (PMA).
The first solvent rinse bath comprising a non-hydroxylic aprotic solvent which is preferably the same solvent as in the first cleaning solution solvent in the category of propylene glycol alkyl ether alkoate selected from the group consisting of propylene glycol methyl ether acetate (PMA), propylene glycol ethyl ether acetate (PGEEA, bp. 158xc2x0 C.), propylene glycol methyl ether propionate (methotate), di(proylene glycol) methyl ether acetate (DPMA, bp. 200xc2x0 C.), ethoxy ethyl propionate (EEP), and di(propylene glycol) dimethyl ether (DMM, bp. 175xc2x0 C.).
The second rinse solvent is a hydrophilic essentially water soluble solvent represented by propylene glycol alkyl ethers selected from the group consisting of di(propylene glycol) methyl ether (DPM, fp 75xc2x0 C.), tri(propylene glycol) monomethyl ether (TPM, fp 96xc2x0 C.), tri(propylene glycol) n-propyl ether, or a mixture thereof, used at a temperature from about room temperature to about 60xc2x0 C.
In the alternative solvent rinse process, the parts after the first solvent rinse in PMA or related non-hydroxylic aprotic solvent are again subjected to the same solvent rinse, preferably PMA in a second solvent bath followed by spray or immersion rinse in IPA, and dried by blowing dry N2 or air on the surfaces followed by heating the component parts at about 90xc2x0 C. to about 120xc2x0 C. for 30 minutes to one hour, preferably under vacuum. In this process, no hydrophilic solvent or water rinse is used.